This invention relates to the fabrication of microlenses and, more particularly, to a process for monolithically manufacturing them as fully integrated components of acoustic ink print heads and the like. Even more specifically, the present invention pertains to a reliable and repeatable process for applying existing semiconductor fabrication technology to the manufacture of microlenses and microlens arrays, thereby facilitating the production for use as part of acoustic ink print heads.
Acoustic ink printing is a promising direct marking technology. It is an attractive alternative to ink jet printing because it has the important advantage of obviating the need for the nozzles and small ejection orifices that have caused many of the reliability and picture element (i.e., "pixel") placement accuracy problems which conventional drop on demand and continuous stream ink jet printers have experienced.
As will be appreciated, the elimination of the clogged nozzles is especially relevant to the reliability of large arrays of ink emitters, such as page width arrays comprising several thousand separate emitters. Furthermore, small ejection orifices are avoided, so acoustic printing can be performed with a greater variety of inks than conventional ink jet printing, including inks having higher viscosities and inks containing pigments and other particulate components.
As is known, an acoustic beam exerts a radiation pressure against objects upon which it impinges. Consequently, when an acoustic beam impinges on a free surface (i.e., liquid/air interface) of a pool of liquid from beneath, the radiation pressure which the beam exerts against the free surface may reach a sufficiently high level to release individual droplets of liquid from the surface of the pool, despite the restraining force of surface tension. To accomplish this, the acoustic beam is brought to focus on or near the surface of the pool, thereby intensifying its radiation pressure for a given amount of input power. These principles have been applied to acoustic printing previously, using ultrasonic (rf) acoustic beams to release small droplets of ink from ink pools.
Prior work has demonstrated that acoustic ink printers having droplet emitters composed of acoustically illuminated spherical focusing lenses can print precisely positioned pixels at a sufficient resolution for high quality printing of relatively complex images. See, for example, commonly assigned U.S. Pat. No. 4,751,529 on "Microlenses for Acoustic Printing", and U.S. Pat. No. 4,751,530 on "Acoustic Lens Array for Ink Printing", to Elrod et al. which are both hereby incorporated by reference.
Acoustic ink printing requires precise positioning of the lenses with respect to each other on very closely spaced centers. In a known manufacturing process the lenses are chemically etched or molded into the substrate. A photolithographic process for isotropically etching them into silicon is described by K. D. Wise et al, "Fabrication of Hemispherical Structures Using Semiconductor Technology for Use in Thermonuclear Fusion Research," J. Vac. Sci. Technol., Vol 16, No. 3, May/June 1979, pp. 936-939 and that process may be extended to fabricating lenses and substrates composed of other chemically etchable materials. Alternatively, it has been suggested the lenses may be cast into materials such as alumina, silicon nitride and silicon carbide through the use of hot press or injection molding processes. However, etching of the spherical lenses into a substrate has been found to be a complex procedure which has not achieved the high reliability and through-put necessary for commercial manufacturing. Furthermore, it has been found that the process of etching the cavities produces some variability in the radius of curvature of the lenses, and in turn this introduces some variability in the size of the ejected droplets. This degrades the quality of the printing. In addition, while hot press and injection molding processes have been suggested, they also have not been shown to provide the necessary reliability and through-put which is necessary.
Therefore, there exists a drawback to the manufacture of acoustic ink printers implementing spherical focusing lenses, due to the difficulty of manufacturing arrays having a high number of spherical lenses. Particularly, an acoustic ink print head will commonly have over a thousand individual ink emitters wherein each emitter has a corresponding lens. It has, therefore, been a further problem to develop a manufacturing process where such arrays can be reliably manufactured to tight tolerances in large numbers.
In view of the foregoing, it has been deemed desirable to develop a manufacturing process which allows for the configuration of microlens arrays for use in acoustic ink print heads, and to develop a process for economically producing such arrays.